This invention relates to a method and apparatus for detecting water impurity in a gas stream. More particularly, this invention relates to a method and apparatus for detecting water impurity utilizing a piezoelectric crystal coated with a metal oxide reactive with water.
At the present time ultrapure gas streams are utilized in chemical reactions such as in the semiconductor industry. These chemical reactions usually are conducted in sealed containers to maintain purity since the gases often times are toxic and are conducted under low pressure in order to decrease the probability of unwanted side reactions. In order to maintain the requisite gas purity, the gas is treated, prior to entering the reaction chamber, in order to remove impurities therefrom. It is general practice to pass the gas through a bed of resin particles which are interactive with impurities such as water in the gas. Over time, the capacity of the bed of resin particles for interacting with the impurities is depleted to a point where breakthrough of impurities from the resin bed occurs and the impurities enter the reaction zone it is difficult to predict when undesirable depletion of resin capacity occurs so that in the absence of independent monitoring means, premature or late removal of the resin is likely. Premature resin removal results in increased resin cost while late removal results in expensive damage to reaction product. It is additionally desirable to have a means for continuously monitoring the gas purity level and providing a measure of gas impurity concentration.
It has been proposed in U.S. Pat. No. 5,138,867 to provide a detection system for sensing concentration of impurities in a gas stream which includes a sensing device which can be hygrometric, spectrophotometric, piezoelectric or colorimetric. The specific piezoelectric device disclosed is a surface acoustical wave (SAW) device. In a SAW device, an acoustical wave is passed along a surface coating on a substrate to measure the change in mass at the interface between the coating and the substrate. Mass change in the coating is caused by reaction of the coating with impurities in a gas which contacts the coating. Accordingly, a reactive polymer coating material is described that is consumed over time and regenerable. In the SAW device, the coating must be thin; on the order of a wavelength of the acoustic wave or thinner in order to permit accurate measurement of impurity concentration. While this device is extremely sensitive to impurity concentration change, i.e. in the picogram level, it is too sensitive for use in a device requiring an extended service, i.e., about one year or more, since the thickness of the coating necessary to have the capacity for extended lifetimes quickly exceeds that which permits accurate measurements. The detection of water vapor using materials like silica gel and alumina on piezoelectric materials was proposed in U.S. Pat. No. 3,385,100. These types of metal oxide coatings absorb moisture through a physisorption, rather than chemisorption, mechanism. Water adsorption on these types of coatings occur principally through dipole bonds (H-bonding) with surface hydroxyls. The approximate water vapor pressure at 25.degree. C. for absorbants like alumina are shown in the table below (Shriver, D. F.; Drezdzon, M. A. "The Manipulation of Air-Sensitive Compounds", 2nd.ed.; John Wiley & Sons, Inc.: New York. 1986; p 72):
______________________________________ Material Water Vapor Pressure (torr) ______________________________________ Molecular Sieves 1 .times. 10.sup.-3 Alumina (active) 1 .times. 10.sup.-3 Silica gel 2 .times. 10.sup.-3 ______________________________________
Absorbants such as alumina, silica gel and molecular sieves exhibit a steady increase in the water vapor pressure as more moisture is absorbed. This continual increase is undesirable, since the sensitivity for moisture steadily decreases as more moisture is absorbed. In addition, due to the reversible adsorption-desorption of moisture from these type of coatings, the coated crystal would need to be located within a carefully temperature controlled environment to eliminate any temperature effects on the physisorption.
A scavenger for oxygen and water vapor impurities comprising metal hydrides are disclosed in U.S. Pat. Nos. 4,950,419 and 4,716,181. However, these devices are not useful for selective reaction with water in an oxygen gas containing stream since coatings of the device are reactive with both oxygen and water.
It would be desirable to provide a means for detecting water vapor in a gas stream which is useful and accurate for extended times. In addition it would be desirable to provide such a means for detecting water vapor capable of quantifying impurity concentration in a gas, particularly a gas stream consisting of or containing oxygen gas.